How the dew point is calculated
The dew point is the temperature the air would have to cool to for its water vapor to condense into dew. This calculator uses the Magnus formula with the constants from Alduchov and Eskridge (1996), b = 17.625 and c = 243.04 °C, with T in °C:
γ = ln(RH ÷ 100) + b × T ÷ (c + T)
Dew point = c × γ ÷ (b − γ)
RH = 100 × eb × Td ÷ (c + Td) ÷ eb × T ÷ (c + T)
Dew point = c × γ ÷ (b − γ)
RH = 100 × eb × Td ÷ (c + Td) ÷ eb × T ÷ (c + T)
The constants were fitted for −40 °C to 50 °C, which covers almost all weather. °F inputs are converted to °C first.
Worked example
At 25 °C (77 °F) and 60% humidity: γ = ln(0.6) + 17.625 × 25 ÷ 268.04 = −0.5108 + 1.6439 = 1.1331. Dew point = 243.04 × 1.1331 ÷ (17.625 − 1.1331) = 16.7 °C, which is 62.1 °F. That's "sticky" on the NWS scale.
How the dew point feels
Relative humidity changes as the air warms and cools during the day, but the dew point stays steady, so it's a better guide to how muggy it feels. The NWS uses this scale for summer:
| Dew point | How it feels |
|---|---|
| 55 °F (13 °C) or lower | Dry and comfortable |
| 55 to 65 °F (13–18 °C) | Becoming "sticky", with muggy evenings |
| 65 °F (18 °C) or higher | Lots of moisture in the air; becoming oppressive |
Limitations
- Below freezing, the "frost point" over ice is slightly higher than the dew point over water; this calculator gives the dew point over water.
- Outside −40 °C to 50 °C the constants are less accurate.
- Your result is only as good as the humidity reading. Cheap hygrometers can be off by 5% or more.